The present invention relates to an electrophotographic recording material comprising an electrically conductive support, a photoconductive layer and, optionally, an insulating barrier layer between the support and the photoconductive layer. The recording material is suitable for repeated or single use in copiers, and for use as a printing plate or a printed circuit.
German Auslegeschrift No. 11 17 391 (corresponding to British Pat. No. 944,126) discloses the manufacture of electrophotographic recording materials, using photoconductive, predominantly low-molecular weight, organic p-type conducting compounds which are sensitized in the visible region of the spectrum by means of (a) suitable, dissolved dyes, as described in German Offenlegungsschrift No. 25 26 720 (corresponding to U.S. Pat. No. 4,063,948), or (b) dispersed photoconductive color pigments, as described in German Auslegeschrift No. 21 08 939 (corresponding to U.S. Pat. No. 3,870,516).
As the charge-carrier generating color pigments, perinones, as specified in German Offenlegungsschrift No. 22 39 923 (corresponding to British Pat. No. 1,416,603) and in German Offenlegungsschrift No. 21 08 958 (corresponding to U.S. Pat. No. 3,879,200), perylene tetracarboxylic acid diimides, as specified in German Offenlegungschriften No. 22 37 539 (corresponding to U.S. Pat. No. 3,871,882) and No. 21 08 992 (corresponding to U.S. Pat. No. 3,904,407), and condensed quinones, as specified in German Offenlegungsschriften No. 22 37 678 (corresponding to U.S. Pat. No. 4,315,981) and No. 21 08 935 (corresponding to U.S. Pat. No. 3,877,935) are used, among others. The above-described systems have in common a double layer arrangement comprised of a thin charge-carrier generating bottom layer containing a high concentration of color pigment and a relatively thick charge transport layer containing an inert biner and an organic p-type conducting photoconductive.
Layer arrangements are also disclosed in which the sensitizing color pigment and the p-type conducting photoconductor are applied, together in one layer, to the electrically conductive support. According to U.S. Pat. No. 3,879,200, for example, the concentration of color pigment required to achieve optimum physical and electrical properties amounts to only 0.1 to 5% by volume of the photoactive layer. On the other hand, the organic p-type conducting photoconductor, comprising aromatic or heterocyclic compounds, must be present in the layer in a concentration of at least 25% by volume, to obtain practicable sensitivities. The binders which are described include electronically inert polymers, such as polystyrene, polyacrylate, cellulose nitrate, polyvinyl acetate, chlorinated rubber, etc.
Moreover, electrophotographic layers are known, which comprise a photoconductive pigment and an electronically inert binder. As the photoconductive pigments, zinc oxide, according to U.S. Pat. No. 3,121,006, cadmium sulfide, according to U.S. Pat. No. 3,238,150, and a number of other inorganic compounds are described. In these layers, charge transport is achieved by a high concentration of the photoconductive pigment. A layer composition of this kind requires a pigment concentration exceeding 50% by volume to permit contact between the photoconductive particles. According to German Offenlegungsschrift No. 32 27 475 (corresponding to U.S. Pat. No. 4,418,134), organic photoconductive pigments can be substituted for part of the inorganic pigment, and for this purpose, pigments such as C.I. Pigment Red 168 and C.I. Pigment Orange 43, which represent derivatives of naphthalene tetracarboxylic acid diimides, have proved suitable. The total proportion of photoconductor in the layer, which is necessary for practical application, is then in the range between 20 and 80% by weight. In view of an application in electrophotographic offset-printing plates, polymers which are decoatable with or dispersible in alkaline solutions are required as the binders.
Because light absorption and charge-carrier generation occur, in particular, in the upper region of the layer, and because transport characteristics are different for electrons and "holes" ("n-type photoconductors"), a good sensitivity of zinc oxide layers is only observed if a negative charge is applied. With positive charging, on the other hand, good sensitivities are obtained in monolayer photoconductor systems which contain, in addition to an inert binder, a metal-free phthalocyanine in the X-form (see German Offenlegungsschrift No. 14 97 205, corresponding to U.S. Pat. No. 3,816,118). The required pigment concentration between 5 and 25% by weight is clearly below the value assumed for contact between the pigment particles.
In an analogous manner, monolayers for positive charging can be prepared from copper phthalocyanine in the .epsilon.-form (Japanese patent application No. 50/38543, published after examination).
From Japanese patent application No. 49/76933, published after examination, it is known that the pigment C.I. Pigment Orange 43 (=C.I. Vat Orange 7) can be converted into a photoconductive form by reacting it with 2,4,7,8-tetranitrocarbazole. The resulting .pi.-complex shows good sensitivities in combination (50:50) with poly-N-vinyl-carbazole as a binder having the properties of a p-type conducting photoconductor.
To achieve high photosensitivities in the case of negative charging, photoconductors in double-layer arrangement are used. But this arrangement has the disadvantage of being produced in two steps of coating application, which is more expensive than the production of a monolayer material. Double-layer arrangements also have the disadvantage of showing an unfavorable residual-charge behavior. Monolayers based on zinc oxide, on the other hand, have low residual charge potentials and can be used for cyclic image reproduction. But due to the high proportion of zinc oxide, layers of this kind show a relatively low mechanical stability and a relatively poor charge acceptance.
Decoatability of the photoconductor layer in the non-image areas, after imaging and fixing the toner image, is a decisive criterion of usefulness in the production of electrophotographic printing plates or printed circuits. As a consequence, photoconductive layers in double layer arrangements containing extremely high pigment proportions are not readily employed in this context. According to published European Patent Application EP-A- No. 0 137 217, double layer photoconductors which are formed of two layers of approximately equal thickness, i.e., a precoating comprising a pigment and a binder and a covering coating comprising a p-type conducting photoconductor and a binder, can be used for the electrophotographic production of offset printing plates, but they are clearly less sensitive than the first-mentioned photoconductor layers, and are also unfavorable from the point of view of production expense.
Monolayer photoconductors containing dissolved sensitizing dyes, as disclosed by German Offenlegungsschrift No. 25 26 720 (corresponding to U.S. Pat. No. 4,063,948), have similar sensitivities but, in contrast to the pigment layers, are sensitive to pre-exposure, i.e., their charge acceptance is noticeably impaired by preliminary exposure. Monolayers containing low concentrations of sensitizing pigments show photosensitivities which are markedly lower than those of double layers and also poorer image reproductions. all of the above-described layer arrangements, however, exhibit unwelcome, relatively large residual potentials after exposure, which potentials rise drastically with increasing layer thickness and lead to difficulties in rendering visible the latent charge image.
For use as electrophotographic resists, monolayers comprising a binder, a dissolved dye or pigment, and a p-type conducting photoconductor are only applicable by laminating processes. Because of the high proportion of photoconductor in such layers, direct application to metals, such as copper or iron, often leads to contamination of the layer or surface and, thus, to a considerably reduced charge acceptance which severely hampers practical use. By means of double layer which do not contain a p-type conducting photoconductor in the pre-coating, these effects can be obviated, but the above-mentioned disadvantage thus appear.